Generally, an electro-photographic color image forming apparatus can be classified according to the image forming method being used into one of a tandem type that forms a full-color image through a single print process and a multi-pass type that forms a full-color image by repeating multiple print processes. Among these types, the tandem-type color image forming apparatus can require multiple light scanning units scanning light beams to form each of several color images, for example, yellow (Y), magenta (M), cyan (C), and black (K) images. Each of the light scanning units can be a device that deflects and scans a beam emitted from a light source across an image receptor along a main scanning direction. A latent image can be formed on the image receptor through the scanning along the main scanning direction while rotating the image receptor in a sub-scanning direction, which is typically perpendicular to the main scanning direction.
In a tandem-type image forming apparatus, the density of color depends on the amount of developer consumed in a print process. Such density can be corrected by adjusting the light output value of a light source of the light scanning units. However, unfortunately, when the light output from the light source changes, not only can the density of the color change, but also a color registration in the main scanning direction can also change. Such a result can occur from a change in the timing of detection of the light by a detecting sensor that detects the light for the purpose of providing the horizontal synchronous signal for the light scanning operations.
The afore-described color registration error may become more pronounced in those tandem-type image forming apparatuses that employ a light scanning unit that forms at least two simultaneous scanning lines with a single beam deflector. Such tandem-type color image forming apparatuses have a first main scanning direction and a second main scanning direction opposite to the first main scanning direction. A color registration error can occur between a first main scanning direction line and a second main scanning direction line based on the change in the light output value.